As a magnetic recording medium such as video tape, audio tape and magnetic disc there has been heretofore widely used one comprising a magnetic layer having a ferromagnetic iron oxide particle, Co-modified ferromagnetic iron oxide particle, CrO.sub.2 particle, ferromagnetic alloy particle or the like dispersed in a binder coated on a nonmagnetic support. However, since such a magnetic material is normally acicular and magnetized longitudinally, it encounters a trouble that it exhibits an increased self-demagnetization and thus cannot provide a sufficient output in the recent requirement for short wavelength recording. JP-A-58-6525 and JP-A-61-273735 (The term "JP-A" as used herein means an "unexamined published Japanese patent application") propose a magnetic recording medium comprising a hexagonal ferrite magnetic material having a magnetizable axis in the vertical direction on the plane. However, since the hexagonal ferrite magnetic material exhibits a poor dispersibility, it, if used as it is, gives a rough surface and hence a space loss, making it impossible to obtain a sufficient output. The hexagonal ferrite magnetic material is also disadvantageous in that it exhibits a small saturated magnetization and thus gives a low long wavelength output.
In the past, many attempts have been made to produce signals having different recording wavelengths at a high output by providing two or more magnetic layers. These attempts are too numerous to mention. For example, JP-B-37-2218 (The term "JP-B" as used herein means an "examined Japanese patent publication") discloses such an attempt. In many of these attempts, the coercive force (Hc), maximum flux density (Bm) and grain size of magnetic material differ from the upper layer to the lower layer for the purpose of providing a high output and a high C/N. Examples of an magnetic recording medium comprising two or more magnetic layers wherein the upper layer comprises a hexagonal ferrite are disclosed in JP-A-60-223018 and JP-A-1-251427. However, since such a magnetic recording medium has a rough surface due to the use of a magnetic material as a lower layer, comprises a thick lower layer and exhibits a wide distribution of coercive force of hexagonal ferrite magnetic material, it cannot provide sufficient characteristics for high density recording.
On the other hand, in the recent tendency for higher density combined with shorter recording wavelength, paying attention to the fact that when the thickness of a magnetic layer is great, a problem grows that self-demagnetization loss during recording and thickness loss during reproduction cause an output drop, the thickness of the magnetic layer has been reduced. However, when the magnetic layer is thinned to about 2.5 .mu.m or less, the effect of the nonmagnetic support can easily appear on the surface of the magnetic layer, giving a tendency for worse electromagnetic conversion characteristics or more dropouts (DO). The effect of the surface roughness of the support can be eliminated by providing a thick nonmagnetic subbing layer on the surface of the support before coating the magnetic layer as an upper layer as disclosed in JP-A-57-198536. However, this approach is disadvantageous in that the head abrasion resistance or durability cannot be improved. This is possibly because that the nonmagnetic lower layer has heretofore comprised a thermosetting resin as a binder. That is, the lower layer hardens, making the magnetic layer to come into cushionless contact with the head and other members. Further, a magnetic recording medium having such a lower layer exhibits a slightly poor flexibility.
In order to eliminate these disadvantages, the lower layer may comprise a non-curing resin as a binder as disclosed in JP-A-63-191315. However, in the conventional process, if a magnetic layer is coated as an upper layer after the drying and drying of a lower layer, the lower layer swells in an organic solvent in the coating solution in the upper layer, causing a turbulence or the like in the upper layer coating solution. This leads to poor surface properties of the magnetic layer that cause deteriorated electromagnetic conversion characteristics or like troubles. Further, the reduction of the thickness of the magnetic layer may be accomplished by reducing the coated amount of the coating solution or adding a large amount of a solvent to the magnetic coating solution to reduce the concentration thereof. In the former approach, the reduction of the coated amount of the coating solution gives no sufficient time for leveling before the magnetic solution thus coated begins to dry. This causes troubles such as coating defect, e.g., patterning of stripes or marks, remarkably reducing the yield. In the latter approach, the reduction of the concentration of the magnetic coating solution causes many voids to occur in the resulting coated film, making it impossible to provide a sufficiently packed magnetic material. These voids also cause various troubles such as insufficient film strength. Thus, JP-A-62-154225 has a great disadvantage that it provides a poor yield.
One of the approaches for resolving these problems is to employ a simultaneous multilayer coating method by which a nonmagnetic layer is provided as a lower layer, followed by the coating of a high concentration magnetic coating solution to a small thickness, as disclosed in JP-A-63-191315 and JP-A-63-187418. This approach provides a drastic enhancement of yield, making it possible to obtain excellent electromagnetic conversion characteristics. However, the recent magnetic recording medium has been required to reach a higher density.
JP-A-2-254623 focuses on switching field distribution (SFD) . This application discloses a magnetic recording medium comprising a plurality of magnetic layers provided on a nonmagnetic support, characterized in that SFD of the uppermost magnetic layer is from 0.4 to 0.7 and SFD of the magnetic layers other than the uppermost magnetic layer are from 0.2 to 0.4. However, if SFD of the uppermost magnetic layer is great, neither 7 MHz output nor 2 MHz output are excellent.
Another application that focuses on SFD is JP-A-2-240824. In this application, SFD of the second (upper) magnetic layer is 0.6 or less. The residual flux density (Br)/coercive force (Hc) ratio (Br/Hc) of the second (upper) magnetic layer is 2.times.10.sup.-6 H/m or more. Further, the central line average surface roughness and coercive force (Hc) are defined.
However, it was found that if Br/Hc is 2.times.10.sup.-6 H/m or more (MKSA unit), or 1.59 or more in CGS unit, there is disadvantageously shown a remarkable drop in 7 MHz output.